As is well known and understood, perhaps the most common method of reducing continuous wave (CW) interference is through the use of bandpass notch filters. However, as applied in audio communications systems and particularly in defense communications systems, the use of such filters has certain inherent limitations inasmuch as passive, low frequency filtering requires relatively large and therefore expensive inductors and capacitors. Additionally, the filtering also tends to distort the voice signal and thus deteriorates its intelligibility. Furthermore, the filtering is implemented around a preset frequency which permits the signal to be jammed merely by changing to jamming frequencies out of the bandpass of the notch filters.
One known type of interference cancelling system is shown and described in U.S. Pat. No. 4,027,264, entitled, "Phase Lock Loop Multi-tone Interference Cancelling System" issued to Frank S. Gutleber, the present inventor, on May 31, 1977. The interference cancelling system of that system employs a phase lock loop which is utilized as a narrow band tracking filter to adaptively lock to interference tones to be cancelled by the process of signal subtraction.
In the above referenced related application entitled, "CW Interference Cancelling System For Spread Spectrum Signals", the CW interference is eliminated by means of a variable delay line and a liner adder which is interposed between a receiver, responsive to orthogonal multiplexed noise codes, and a passive matched filter and linear adder which operate to time compress the coded signals to a lobeless signal. The delay line operates to provide an inverted interference signal from an input interference signal which is exactly 180.degree. out of phase with respect to the input interference signal. By adding the delay signal to the input signal in the linear adder, a total cancellation of the interference signal is provided while the signal level of the noise coded communications signal at the output of the receiver remains unchanged.
The present invention is particularly adapted for use with communications systems utilizing noise coded signals. Such systems, furthermore, are well known and are particularly desirable because they exhibit an immunity against interference and jamming. Noise coded means that information is coded with a code that is "noise like" in that it will compress to an impulse when detected with a matched filter. In particular, one class of such noise codes are known wherein pairs of coded signals termed "code mates" have autocorrelation functions which provide a peak output at a given time and a zero output or outputs having the same magnitude but opposite polarity at all other times. For example, when the code mates are multiplexed, matched filter detected and linearly added, there is provided a lobeless impulse output of a relatively high amplitude at a given time and a zero output at all other times. Such codes and systems utilizing such codes are typically shown and described in U.S. Pat. No. 3,461,451, entitled, "Code Generator To Produce Permutations Of Code Mates", which issued to Frank S. Gutleber on Aug. 12, 1969; U.S. Pat. No. 3,634,765, entitled, "Systems To Provide An Impulse Autocorrelation Function . . . Of One Or More Of Said Code Signals", which issued to Frank S. Gutleber on Jan. 11, 1972; and U.S. Pat. No. 3,908,088, entitled, "Time Division Multiple Access Communications System", which issued to Frank S. Gutleber on Sept. 23, 1975.